fix: early return when validating empty email string

[karrui]

Problem

Fixes bug where optional email fields are still failing validation for restricted email domains even when they are empty.

Closes #431

Solution

Bug Fixes:

• Remove restricted email domain field validation when field is an empty string (on required fields, the required validator will still fire)

After Screenshots

AFTER:

Required fields will have error when field is empty when submitting

Optional field will have no error when field is empty when submitting